Induction furnace



Oct. 3, 1944. BENNETT 2,359,285

INDUCTION FURNACE Filed July 17, 1942 5 MUM Lilli 11,? a: I 51 WM: #7\ it ATTORNEY Patented Oct. 3, 1944 INDUCTION FURNACE Edward Bennett, Madison, Wis., assignor to A'. 0.

Smith Corporation. Milwaukee Wis., a corporation of New York Application July17, 1942, Serial No. 451,282

3 Claims.

This invention relates to an induction furnace for heating tubular conductive articles by magnetically'inducing alternating heating currents eircumferentially of the article.

The invention has been applied in the heating of aerial bomb casings preparatory to forging operations for closing the forward end portion of a tube to form the nose of the bomb. In such forging operations, as, for instance, described in the application of Warren F. Heineman, Serial No. 410,833,'fi1'ed September 15, 1941, for Method of making bomb shells, it is desirable to heat only a predetermined 'end portion of the blank, leaving a fairly sharp line of heat gradient between that and the cooler body of the blank. This requires that the heating operation be fast 'so as not to obtain too much conduction of heat to the colder part of the blank. In practice the end of the blank is heated to the required forging temperature in a fraction of a minute.

Diillculties arise in heating a blank in so short a time. High frequency induction heating was found to be unsatisfactory for heating. thick tubular bodies such as are employed invmaking bomb casings, due'to the tendency to heat the metal on the surface and not to effectively heat that beneath the surface. With high frequency heating the metal at the surface of'the blank may beraised to the melting point while that a short distance beneathisstill cold.

Another difficulty in applying induction heating to partially formed blanks in' making bomb casings lies in the fact that where substantially all of the magnetic flux passes axially through the center of the part being heated, the difference in diameter of different portion of the section being heated leads to a difference in resistance to the flow of the induced current between the portion of smallest diameter and that of largest diameter resulting in alack of uniformity of heating between the smaller end portion of the blank and the larger body portion in proportion to the FR values in the two portions, In order to satisfactorily forge a blank in mak ing bomb casings, there should 'bea substantial .the core I and coils uniformity in temperature between the different portions of the art of the blank being forged.

The principal object of the present invention is to obtain a more uniform or a controlled nonuniform heating of the section of the blank to be forged or heated.

and arranged to obtain a differential, distribution of magnetic flux axially of the article more nearly proportional to the differences in resistance of the several circumferential 'portionsof the article being heated, or, in the event a predetermined nonuniformity of heating is desired, a differential distribution of magnetic flux axially of the article to obtain the same. I An embodiment for carrying out the invention is illustrated in the accompanying, drawing in which:

Figure 1 is a vertical central section through a furnace andshowing the blank in pcsition for heating; and

Fig. 2 is a top away.

plan view, parts being broken The furnace comprises, in general, the magnetlc core structure I and energizing coils 2.

The core structure I comprises outer vertical legs 3 with upper horizontal legs I extending inwardly toward the center, and similar lower honzontal legs 5 extending inwardly iacent the tubular blank 6, being heated. An axml or central leg I is disposed vertically within the upper end of the blanle. 6 and bridging the gap between ,the .upper and lower legs 4 and respectively, to-provide a path for the magnetic flux produced by the energizing coils I.

The coils 2 are of the-flat type comprising a number of flat turns 8 of copper tubing encircling the blank I in substantially close proximity thereto and connected to a suitable source 9 of low frequency alternating current. A similar gr up of coils II is disposed inside of the blank to e circle the central core leg 1. A vertically movable pedestal il supports the inner coils l0 and ing operation of the furnace and a heat insula- 1 12 is laced between the blank 6 and tion she I p 2 to prevent radiation of heat from the blank to the coreand coils.

The blank'i is illustrated as the upper-end of a tubular bomb casing after it has been partially formed with its partially closed' end uppermost.

pedestal H and then ver the It is first placed 0 ace through raised vertically with it into the end of the blank 5 extends to a position near the upper horizontal le s I of the core.

The cycle of the current employed should be below 500 alternations or pulsations per second to a position adthe lower leg 5 of the outer and preferably is 60 cycles per second since this cycle is readily supplied from commercial current sources without the addition of costly equipment.

The current should be of single phase and instead constituted of. a single series of longitudinally extending laminations as shown, or the laminations may be arranged in quadrants to extend in a more nearly radial direction.

In carrying out the invention, the iluxtravers i ing the central leg '1 at the upper smaller diameter end ofthe blank 6 should be less than the flux traversing the same leg lower down, where the blank is nearly its largest diameter. This is accomplished by forcing a leakage path for the flux through the walls of the blank as illustrated by the arrows l3, the main flux path through the core structure being illustrated by the arrows It.

A gap i is provided. between the inner edge of the upper leg 4 and the circumference of the central leg I of the core directly. through the shorter path It which is nearer to the energizing coils 2 and which bleeds flux from the upper end of the, central core leg ll. This reduces the flux passing through the upper smaller end of the blank 8 and therebyreduces its rate of heating to more nearly that of the lower portion of the section of theblankbeing heated.

Another factor which should be considered and which may vary depending upon the shape or the article, is the location of the energizing coils 2. 45

The exact location of the coils 2 and the number of turns therein will depend uponthe shape of .the article being heated. In the drawing the coils} are shown disposed near 'the lower end of the furnace, leaving the upper end substantially free of energizing coils. If the blank were cylin- Y drical it would be'desirable tov distribute the coils all along the section being heated-in order to obtain uniform heating conditions. Howevenior blanks of substantially uniform thickness, as'the -upper end of the blank getssmallerin successivef stages oi forging-more and more flux should be passing through the smaller enddiverted from of the blank and! yet caused to pass through the l rger end. y

the need. The respective" coils-may eitherbe dropped out by means of switches or successive Where the blank gets thicker 1 or su throughthe nose, and makes-s lesser dish nbn' ent oithe primary coilstoward the larger diameter end of the blank In such case, the coils can beplacedpearer to the lopsistructure. The high reluctance of the gap l5 tends toi'orce stray flux ropping the coils away from the so upper end .of the blank a little farther for each successive heating audio verting of the-fluxes more nearly proportional :to.

' operation the dii urnaces may be employedhavlng' the diiferent- 'coil distributionsdescribed. 5. asthe end-is contracted in diameter. the greater counter-mag- ,netomotive force or the greater current in; the thickened nose opposes-the establishment tudinal'center of the, blank portion being heated to, compensate for the raising of the center .01

area of the cross section taken on the longitu dinal axial plane of the blank wall. The loca- 5 tion of the coils may also depend upon whether the process being employed is one of successive reheating with the utilizing of a conservation of heat from one step to another, as set forth in I applicants co-pending application Serial No.

451,281,,flled July 17, 1942. .Where considerable conservation of heat is obtained, as distinguished from heating a relatively cold blank, the added thickness of the nose may not require any additional ampere turns to heat the nose within the time provided for the heating cycle.

In general the resultant magnetomotive force of the primary coils should have its center oflset from the center of area of the cross section taken .on the longitudinal axial plane of the blank wall ameter.- This ofl'setting of the center of the primary magnetomotive force, together with the gap l5 eifects a reduction in the'flux threading the small end of the blank and in the circumferential voltages induced in the shorter heating current paths in the small end. As a result the induced current densities are kept at the desired uniformity over the entire section of the heated end, and the heating energy supplied to the nose portion of the article is prevented from becoming excessive.

Either the oifsetting of the center of primary magnetomotive force or the employment of a gap in the core structure may be sumcient in. a

given instance, but it is preferred toemploy both means of limiting the flux at the small end of the blank, the former means enticing a leakage of flux through the wall of the blank and the latter compelling such leakage by reason of the in- -40 creased reluctance at the end of. the core structure farthest from the coils.

' The placing of the primary coils 2 at the bottom of the furnace, immediately adjacent the leg 5 of the core structure I, serves to effect a high flux density through the wall of the blank at this location and to cut on the flux from passing downwardly, As a result there is a very sharp line of heat gradient in the blank at this point. By extending the core structure up- 'wardly and. around the end of the blank substantially above the coils, there is a lower flux density radially through the wall of the blank above the coils and a more readily controlled heating effect in this region.

The invention constitutes a practical solution for the uniform" heating of tubularsectionsof' varying diameter, and its principles may be em-' -;ployed to obtain either a substantial uniformity nonuniiormity or heating thereof.

.In some cases the upper leg, I of the core structure may be closed While the inventionhas been described as applied to the heating of. a-tapered tubular blank ofnonunii'orm diamete 1 the heating of.other types of blanks including tubes of unii'orm inside or outside diameter and tapered in thickness. It is also applicable where a predetermined nonuniformlheat'ing of a, cylin drical tube or other shaped blankis desired.

that is to be heated, toward the end of largest di-- of heating of such a blank ora predetermined in the center and the upper. end of leg 1 be dropped to provide the air gap. or added reluctance between itand the leg 4.

,' it is also applicable to Vari ous embodiments offthe invention may be- I6 employed within the scope or the claims; I Y

The invention is claimed as follows: 1. In low frequency electric induction heating of tubular blanks, the method of compensating for difierences in resistance to the circumferential flow of heating current in differ ent' adjacent longitudinal regions of the blank being heated where a central core element passes axially through said regions to provide a flux path of high permeability separate from the blank, comprising offsetting the center of the magnetomotive force of the primary winding longitudinally toward the region of higher resistance to promote the diversion of flux from said central core element radially through the wall of the blank in the region of lower resistance and thereby provide a greater amount of axial flux passing through the core in the region of the'blank of higher resistance than in the region of lower resistance. A

2. In low frequency electric induction heatin of tubular blanks, the method of compensating for difierences in resistance to the circumferential flow of heating current in different adjacent longitudinal regions of the blank being heated where a central core element passes axially through said regions to provide a flux path of high permeability separate from the blank, comprising offsetting the center of the sistance, and providing a high reluctance in the flux path at the' end of the lower resistance region opposite the center of magnetomotive force, to promote leakage of flux from said central core element radially through the wall of the blank in the region of lower resistance of the blank and thereby reduce the amount of axial flux passing through the core in said region.

3. In low frequency electric induction heating of tubular blanks, the method of compensating for differences in resistanceto the circumferential flow of heating current between a partially closed end portion and an adjacent body portion to be heated where a central core element passes axially through said regions to provide a flux path of high permeability separate from the blank, comprising providing a high reluctance in the flux path at the end of the core element adjacent the partially closed end of the blank to promote by-passing of said partially closed end of the blank by the flux and thereby reduce the amount of flux passing axially magnetomotive force of the primary winding longitudinally toward the region of higher rethrough the core element in the region of the partially closed end of the blank as compared to the amount of flux passing axially through the core element in the region of the adjacent body portion of the blank.

EDVVAF: EENNETT. 

